The present invention relates to a hydraulic control apparatus and method for an automatic transmission, and specifically, relates to a hydraulic control apparatus and method for suitably controlling a working oil pressure to be supplied to the automatic transmission under engine low temperature condition, for example, −20° C. or less.
There has been proposed a hydraulic control apparatus for a belt-drive continuously variable transmission, which includes an oil pump driven by an engine through sprockets and a chain therebetween to discharge a working oil. The working oil discharged from the oil pump is fed to a pressure regulator valve to set a line pressure that is supplied to the automatic transmission. A solenoid-operated valve is connected with the pressure regulator valve. A control unit outputs a command to the solenoid-operated valve so as to produce and transmit a signal pressure to the pressure regulator valve. In response to the signal pressure, the pressure regulator valve sets the line pressure. The line pressure is set to a minimum value immediately after engine start and to a target value raised as the engine speed increases after the engine start.
In the conventional art as described above, there occurs overshoot in the line pressure when the line pressure abruptly varies from the minimum value immediately after the engine start to the target value raised in accordance with increase in engine speed after the engine start. FIG. 3 illustrates the overshoot in line pressure PL. In addition, the overshoot becomes significant when a vehicle driver operates an accelerator to rapidly increase the engine speed after the engine start, thereby causing abrupt increase in amount of the working oil discharged from the oil pump. Further, there occurs more significant overshoot in line pressure PL upon start of the engine in a low temperature condition thereof, for example, at the temperature of −20° C. or less. FIG. 4 illustrates the overshoot under the low temperature engine condition. The overshoot as shown in FIG. 4 will cause an excessive pressure to be applied to pulleys of the continuously variable transmission, and an overload to the chain for driving the oil pump. This requires an expensive chain capable of withstanding the overload.
Japanese Patent Application First Publication No. 2004-124959 (corresponding to U.S. Patent Application Publication No. 2004/0063523 A1) describes a method of controlling a line pressure in an automatic transmission in order to suppress overshoot in the line pressure. In this document, at engine start in a low temperature condition, when the actual line pressure equal to or greater than a preset value continues for a period longer than a preset time, it is determined that line pressure PL is in a regulated state, and a command pressure for controlling line pressure PL is maximized.